In orthopedics it is known to use various types of reconstruction assemblies to repair bone joints that have become deteriorated, damaged or degenerative, such as due to trauma or disease. Some reconstructions involve the use of various components such as bone screws, plates, bone grafts, fusion implants and other components. Depending on the type and method of reconstruction selected, complete stabilization with no movement may be selected, or a predetermined amount of controlled movement may be selected. In one technique of spinal reconstruction, for example, fusion of adjacent vertebrae is achieved using one or more plates fastened to adjacent vertebral segments in order to join the vertebral segments in a predetermined relationship for stabilization, sometimes installing a fusion device such as an implant or bone graft.
While complete fusion and, thus, resultant loss of movement between adjacent vertebrae is sometimes prescribed, fusion does limit movement and in the long term may adversely affect the disc adjacent to the fused joint by imposing heightened stress and wear. An alternative to fusion using motion preservation devices restore significant motion and disc space height which minimizes stress concentrations and pain.
The various known systems for allowing controlled movement of joint reconstructions have shortcomings. Such shortcomings include lack of versatility so as to require multiple configurations and sizes of hardware on hand during surgery; prohibitively complex or expensive components; lack of anatomical correspondence with resultant poor fit; high stress concentrations and unnatural load forces on adjacent or fused bone segments; and other shortcomings. Known motion preservation devices are generally restricted to only very stable constructs and degenerative disc disease cases, which is only 5%-10% of all cases.
Known designs include an insert that is positioned between adjacent vertebrae and that contacts the vertebrae only on the end plates, where the load is transferred in an axial direction parallel to the axis of the vertebral column. Such designs do not provide adequate initial stability and thus are subject to, for example, lateral slide out of the insert. In some cases this problem could be addressed by implementing additional components such as plates or artificial ligaments, thereby increasing cost, complexity, surgery time, and invasiveness.
Other known designs include structures having sections that overlap end faces of adjacent vertebrae, but do not transfer axial load from the end faces. Instead, they have additional sections that are fastened to anterior or other portions of the vertebrae using bone screws or similar means, thereby supporting the load via the screws and the interior sections. This type of system not only causes high stress concentrations in and around the bone screws and their anchoring points in the vertebrae that are at risk for failure under load, but such a system prevents or shields axial load transfer through the end plates. By preventing load through the end plates, bone on-growth is significantly prevented.
Various known assemblies require a variety of sized sets to be on hand during surgery so that a surgeon can make a determination during the procedure as to which size will be best suited for a patient. This adds to the cost or reduces the versatility of the known assemblies.
Various known assemblies require a relatively large amount of surface area of a vertebra to the extend that constructing multiple, adjacent levels of reconstructed vertebrae (i.e., spanning two or more adjacent vertebral disc spaces) is prohibited simply because there is not enough space to install all of the components required. Other known designs include keel sections that are placed into channels cut into vertebral end faces. Such keel designs are subject to increased risk of cross-fracture of the vertebrae because they require channels to be cut which remove bone material crucial to structural integrity, and they often leave no room for additional implants such as on the opposite side of a vertebra.